This invention concerns improvements in or relating to ceramic welding processes and in particular has reference to the size characteristics of the powder elements of the composition of matter employed in such processes.
The present invention also has reference to an improved composition of matter for use in ceramic welding processes.
Such processes are well established for repairing the internal refractory structures of coke ovens, glass furnaces and the like and are disclosed for example in British Patents Nos. 402 203, 1 330 894 and 2 035 524 and Swedish Patent No. 102 283. A ceramic welding process usually involves the conveyance in a carrier gas of a composition of matter in the form of a mixture of powders to a lance, the powder particles being entrained within the lance in an oxygen-containing gas and projected from the lance to a surface where a part of the powder mixture reacts exothermically with oxygen to cause at least partial fusion of the other part of the powder mixture, both with itself and with the surface, so that a ceramic weld is formed.
A composition of matter suitable for use in a ceramic welding process is typically a mixture of refractory oxide and metal and/or metalloid particles in powder form. A composition of matter and process for use in forming refractory masses by a ceramic welding process are disclosed in British Patents Nos. 2 154 228, 2 110 200 and 2 170 191.
British Patent No. 2 170 191 specifies the size characteristics of the refractory particles and the oxidisable motal particles by various properties including one term which approximates to average size and another term which relates to size distribution. The language of the principal claims of 2 170 191 can be simplified by translation into more readily understandable expression to convey the meaning that the average size of the refractory particles must be greater than the average size of the oxidizable particles and that the spread of sizes of the refractory particles should be above a certain minimum. The former of these two requirements is plainly obvious and merely reproduces the disclosures of earlier patents in the ceramic welding field, for example British Patent No. 1 330 894 which gives maximum average sizes for refractories and oxidizable particles as 500 microns and 50 microns respectively. A wide distribution of sizes, giving a high size range spread, is said in 2 170 191 to be of benefit in that it contributes to the formation of durable welds which are less porous and contain less cracks than welds produced using refractory particles of more homogeneous size distribution.
In view of the benefits said to be derived from the operation of the process and use of a composition of matter as defined in British Patent No. 2 170 191, we have found, surprisingly, that high quality, durable refractory masses can be formed by employing refractory particles of more uniform size distribution than is suggested to be the case by this earlier patent. Moreover, the removal of the coarse fractions of the refractory particles assists the flow of the composition of matter through the transport system, enables smoother finish to the resultant weld to be achieved and reduces the proportion of conveyed material which does not form part of the weld. The elimination of the fine size fractions of the refractory particles is also beneficial in aiding the flow of matter, but has added benefits in regard to reducing the extent of silica dust which becomes airborne during handling, to reducing the dust cloud at the reaction zone and hence to reducing the proportion of conveyed material which does not form part of the weld, and to enhancing the exothermic reactions of the oxidizable particles by not stifling these reactions in a similar manner to stone dust which is used in extinguishing fires in underground mines.
The earlier British Patents Nos. 2 154 228 and 2 110 200 describe compositions of matter consisting of incombustible refractory material and particles of exothermically oxidizable material which latter have a characteristic average size of below 50 microns and comprising silicon and aluminium, the aluminium being present in an amount up to 12% by weight of the total mixture. Claim 8 of British Patent No. 2 154 228 prescribes the aluminium content as being at least 1% by weight of the total mixture. In the Examples of these patents, mixtures of silicon and aluminium are described as being used with refractory oxide particles comprising one or more of the following namely zirconia, magnesia, alumina, silica, sillimanite and mullite. It is a feature of all the aforementioned patents that a suitable choice of refractory particles can be made to give a weld repair composition which is as close as possible to that of the substrate refractory to be treated when this is desired.
The ceramic welding process is widely used in the repair of coke ovens the linings of which are formed from silica refractory blocks. Conventionally, the composition of matter, is the welding powder, comprises a mixture of aluminium, silicon and silica particles, and Example VIII of British Patent No. 2 154 228 gives a composition of matter for the repair of a coking furnace as comprising 1% aluminium, 12% silicon and 87% silica (% by weight). (It is to be noted that the Example in question actually quotes 80% silica but it is believed to be an erratum).
In coke ovens, the silica refractory blocks are not 100% pure silica and contain inter alia small quantities of aluminium, iron and calcium impurities present in complex refractory oxide molecules. Furthermore, the silica is present in the crystalline forms tridymite and oristobalite. The preferred silica component of the welding powder is crushed silica refractory of the same crystalline and chemical form as indicated above.
Use of a welding powder mixture comprising aluminium in an amount of at least 1% by weight of the total mixture, silicon and crushed silica refractory cannot always produce a weld composition which matches exactly the composition of the silica substrate. The chemical mismatch is compounded by the fact that a certain proportion of the applied welding powder mixture is lost at the periphery of the spray of material projected from the lance, and does not form part of the final weld since it is not accommodated within the hot zone at the repair area. The lost material is essentially refractory and thus the use of aluminium, particularly in amounts of at least 1%, in the welding powder, results in significantly higher quantities of aluminium as an oxide in the weld repair per se than is present in the substrate refractory silica brick.
Furthermore, it has been found that welds formed from powder mixtures of aluminium, silicon and silica may contain a proportion of silicon metal which is considered at least by some skilled in the art to be detrimental to the quality and durability of the resultant weld, and hence to the efficacy of the process as a whole.
It is also to be noted that when repairing coke ovens the temperature of the refractory can be 800.degree. C. at the ends and possibly 1200.degree. C. and in excess thereof at the centre of the oven. While it may be the case that the use of aluminium in the welding powder mixture can promote good initiation of the weld at lower temperature, it can lead to high and undesirable reaction rates at relatively higher temperatures, namely 1200.degree. C. and above, thereby resulting in overheating of the reaction zone magma layer which becomes low in viscosity and flows easily. When this phenomenon occurs, correct application of the weld becomes difficult since the forward velocity of the projected welding powder forces the magma layer to spread and distort at the periphery, thus producing poor quality welds which are porous and have an uneven finished surface.